The invention relates to a fixed-axis multibladed anemometer having the capacity of accurately sensing the speed component of air flow along the axis of rotation of the instrument at all angles of the incidence of the moving air.
Anemometers utilizing helicoidal blades have been in use for many years. The helicoidal shape of the blades assures a constant angle of attack all along the blades as they rotate. The helicoidal blade form has been used in previous fixed anemometers, such as those shown in the copending application filed by the inventor herein and Clifford J. Auvil, Ser. No. 680,650, filed in the U.S. Patent and Trademark Office on Apr. 27, 1976, entitled "Environmentally Resistant Anemometer".
In the prior art the mathematical derivation of these blade forms assumes that the blade is infinitely long and neglects the finite length to the tip or outer end of the blade, the hub effects and the interactions between the blades. The vortices that are generated at the blade tips and the vortices generated at the inner sections between the blades can cause significant errors in fixed axis anemometers. In a typical four-bladed anemometer having flat helicoidal blades, the error is approximately 10 to 20 percent where the angle of incidence of the wind to the fixed axis is between 60.degree. and 90.degree.. This error source can be so large so as to cause the instrument to rotate in a direction apparently counter to the wind direction component for odd numbers of blades at large angles of incidence, particularly for large numbers of blades (more than thirteen) and for blades designed for a 45.degree. helicoidal twist, but set at a different angle. This occurs because of loss of aerodynamic symmetry, or a change in the portion of the anemometer blades interacting with the flow.
The errors in these fixed axis anemometers have been attributed to the von Karman vortices shed by the shaft on which the anemometer is mounted. A variety of devices, such as shaft extenders, surface roughening, and trip wires on the shaft have been applied to mitigate the errors, but these methods have failed to solve the problem. According to the present invention, it has been recognized that the source of the errors is in the vortices generated by the blades. In the application, Ser. No. 680,650, an approximately spherical hub was used to generate a dynamic pressure flow. This flow was directed with two parallel circular ridges on the surface of the hub and a rectangular channel having a centrally positioned post therein at the base of the anemometer blades was provided to cause air to flow along the blades to displace the blade tip vortices away from the ends of the blades. It was found that this structure was not reliable at all wind speeds, directions of wind change, and angles of incidence because of the small eddies of atmospheric turbulence which induces instabilities in the flow over the spheroidal or ellipsoidal hubs and in the flow over the flat blades. The instability occurs because a sphere and ellipsoid are three dimensional aerodynamical bodies which have separation points and flow irregularities occur even under conditions of low turbulence and constant air flow, said instabilities propagate into the air flow over the flat-plate blades.
Flow separation over flat plates is very sensitive to the level of turbulence in the flow, and the angle of attack of flat blades to the flow. Changes of 1.degree. in the blade set angle in flat plate anemometers cause 10 to 20% errors at critical angles. This makes the instrument sensitive to turbulent flows.